This invention relates to a fiber bundle composed of a plurality of optical fibers and a method of manufacturing the fiber bundle.
This invention relates also to a laser apparatus and a laser machining apparatus, each apparatus using the fiber bundle.
In the fields of optical communication and laser machining, it is desired to develop a laser apparatus which is greater in output power and is less expensive. An optical fiber laser apparatus is known for its greater potential to meet the above-mentioned demand.
In the optical fiber laser apparatus, a single transverse mode of laser oscillation can relatively easily be achieved by appropriately selecting a core diameter and a difference in refractive index between a core and a clad. In addition, by confining a light beam at a high density, it is possible to enhance an interaction between a laser active material and the light beam. Furthermore, by increasing the length of an optical fiber, the interaction can be extended so as to produce a high-quality laser beam with a high efficiency. Because of the above-mentioned superior characteristics, it is possible by the use of the optical fiber laser apparatus to obtain at a relatively low cost a laser beam excellent in quality and having a transverse mode free from an influence of the intensity of laser output, heat, and vibration.
In order to realize further increase in output power and efficiency of the optical fiber laser apparatus, it is necessary to efficiently introduce an excitation beam into the optical fiber in a region (typically, a core) where laser active ions, pigments, or other luminescent centers (hereinafter referred to as the "laser active material") are doped. Generally, when the core diameter is determined so as to satisfy a single-mode waveguide condition, the core diameter is restricted to a value not greater than ten and several micrometers. Therefore, it is generally difficult to efficiently introduce the excitation beam within the core diameter. To overcome the difficulty, proposal is made of a so-called double-clad fiber laser (for example, disclosed in H. Zellmer, U. Willamowski, A. Tunnermann, and H. Welling, Optics Letters, Vol. 20, No. 6, pp. 578-580, March, 1995).
The double-clad fiber laser comprises a core, a first clad surrounding the core and having a first refractive index lower than that of the core, and a second clad arranged outside of the first clad and having a second refractive index lower than the first refractive index. With this structure, an excitation beam introduced into the first clad is kept confined within the first clad during propagation because total internal reflection occurs due to the difference in refractive index between the first and the second clads. During the propagation, the excitation beam repeatedly passes through the core to excite the laser active material contained in the core. In the double-clad fiber laser, the excitation beam is introduced into the first clad. The first clad has a sectional area corresponding to several hundreds to one thousand times that of the core. Therefore, a greater quantity of the excitation beam can be introduced so as to increase the output power.
Thus, the double-clad fiber laser is advantageous in that the oscillation efficiency is high and that the transverse mode of oscillation is a single mode and stable. Therefore, the double-clad fiber laser has a high ability as a machining laser for fine cutting or fine welding.
However, the double-clad fiber laser is disadvantageous in that its laser output is restricted due to the increase of loss resulting from the nonlinear effect such as Brillouin scattering and Raman scattering at the core and to the damage by intense light at the core. With a core material currently available, the double-clad fiber laser is restricted in output power to a range between several tens watts (W) to hundreds and several tens watts (W).
In order to overcome the above-mentioned disadvantage, it is a straightforward idea to increase the core diameter. However, the increase in core diameter of the fiber laser inevitably results in multiple modes of laser oscillation. In case of the multiple modes, the stability of the transverse mode as the advantage of the fiber laser will lost. In this event, the transverse mode of the laser output readily varies due to the intensity of the output, slight vibration of the fiber, and the change in shape. For example in laser machining, this results in unstable distributions of light intensity at a condensing point.
As another approach to compensate the disadvantage of the double-clad fiber laser, it is proposed to use a fiber bundle (a bundle of fibers). This is because a bundle of a plurality of fiber lasers of a single transverse mode provides the increase in output power corresponding to the number of the fibers.
However, if a plurality of fiber lasers of a single transverse mode are simply bundled, each core is surrounded by a clad far greater (about 100 times in diameter) than the core. Therefore, even if the fiber bundle is used in a laser apparatus, the cores as emission points are dotted in a wide spread so that the luminance is decreased.